1. Field of the Invention
The present invention relates generally to a fuel supply control system for an internal combustion engine. More specifically, the invention relates to a fuel supply control system which provides satisfactory engine stability at a low engine load condition, such as an engine idling condition.
2. Description of the Background art
In the modern automotive internal; combustion engine, the amount of fuel supply is precisely controlled according to engine driving conditions. Generally, fuel supply amount is determined on the basis of an engine revolution speed and an engine load condition. Intake airflow is used as a typical engine load condition representative parameter. As is well known, one of typical processes, such as that in D-Jetronics type fuel injection control system, a basic fuel supply amount is generally derived on the basis of the engine speed and the intake air flow rate. Though such type of fuel supply control is popular in the modern automotive internal combustion engine, sensors for monitoring the intake air flow rate as the engine load condition are relatively expensive.
For facilitating cheaper fuel supply control, intake air pressure (boost pressure) is used as an engine load representative parameter in certain fuel supply control systems, such as D-Jetronics type fuel injection control. Since pressure sensors for monitoring boost pressure in an air induction system is relatively cheap in comparison with the intake air flow rate sensors. In such a fuel supply control system, basic fuel supply amount is derived generally based on the boost pressure. The basic fuel supply amount is corrected by a correction value derived on the basis of correction factors including an engine speed.
In D-Jetronics type fuel injection control, boost pressure in the air induction system is varied with a certain lag relative to variation of the engine speed. This lag may affect the precision of fuel delivery. Basically, the degree of influence to precision is approximately proportional to the fluctuation rate (new engine speed/old engine speed) of the engine speed. Since the fluctuation rate is maintained substantially small at relatively high engine speed condition, influence to precision in control of fuel supply control is relatively small and cannot raise serious problems. However, on the other hand, at the low engine speed condition, the fluctuation rate of the engine speed becomes substantial to cause degradation in precision of fuel supply amount control. This tends to cause an unstability of engine, increasing possibility of engine stalling. Particularly, possibility of causing engine stalling becomes high while the engine is coasting at neutral gear condition due to falling air/fuel ration to a too lean condition.
In order to prevent the engine from stalling, Japanese Patent First (unexamined) Publication (Tokkai) Showa 57-68544 proposes fuel supply control in an engine idling condition, in which engine speed difference is differentiated to adjust the fuel supply amount on the basis of the differentiated value. In addition, spark advance is adjusted on the basis of the differentiated value. On the other hand, Japanese Patent First (unexamined) Publications (Tokkai) Showa 60-203832 and 60-128947 proposes adjustment of the fuel supply amount on the basis of engine speed difference or engine speed difference and variation magnitude of boost pressure.
Such prior proposed fuel supply control systems require substantially complex arithmetic operations. Furthermore, the precision level or response characteristics in air/fuel ratio control cannot be satisfactory.